1. Field of the Invention
The present invention is in the field of liquid crystal display devices and liquid crystal compositions which are responsive to irradiation by laser beams to produce thermowriting.
2. Description of the Prior Art
There are in the prior art liquid crystal display devices including the so-called static image liquid crystal display device of high resolution, in which laser beams modulated by a signal based on an optical image to be displayed, e.g., a video signal, are irradiated onto a liquid crystal cell to thereby write the optical image to be displayed into the liquid crystal cell. Thereafter, light is irradiated onto the liquid crystal cell to project the optical image onto a screen. Specifically, the liquid crystal in such a display device exists, for example, in the smectic phase normally and undergoes phase transition into the nematic phase and later into the liquid phase upon irradiation by the laser beams, due to the effect of heat. When the heated portion of the liquid crystal is quenched after the irradiation by the laser beams, the molecules of that portion do not revert to their unidirectional orientation but form a disturbed texture such as the so-called focal conics and the like, and image writing is conducted utilizing this texture which intensely scatters light. The written image can be erased by restoring the texture to the smectic phase in which the molecules are oriented unidirectionally as in its initial state by the application of an external electric field.
The light scattering texture formed by the heating effect of the laser beam irradiation is maintained without destruction so that the image is preserved, that is, memorized, unless an external electric field is applied or a high external temperature is again applied.
In such a liquid crystal display device, it is necessary for actual image display that the rewriting for the static image be made rapidly. To enable rapid rewriting, it is required that the heat conversion efficiency be increased by efficient light absorption of the writing laser beams in the liquid cell. In view of the above, it has been proposed that a liquid display device of this kind should incorporate a dye having a high light absorbance for the wavelength of the writing laser beams such as semiconductor laser beams, for example, at 780 nm.
As disclosed, for example, in Japanese Laid-Open Patent Specification No. 197485/1984 textures causing the scattering of light upon writing by taking a vertically oriented mode, i.e., the focal conic textures, are formed as small-diameter spots in the liquid crystal and the dye added thereto in a liquid crystal cell, so as to obtain a higher resolution power and to improve the absorbance of light energy due to the addition of the dye.
As described above, it has been proposed to add a dye into the liquid crystal of a liquid crystal display device. A squarylium type dye, for example, NK-2772 (trademark of a product manufactured by Nippon Kanko Shikiso Institute), is mixed with about 0.35% by weight of the dye (referring to Japanese Patent Laid-Open Specification No. 197485/1984). In this case, as the concentration of the dye in the liquid crystal is increased, the entire absorbance for the laser beams can be improved and the light-to-heat conversion efficiency can be enhanced to improve the writing velocity. However, it is actually necessary to limit the amount of dye added to the liquid crystal to less than the level corresponding to the solubility limit of the dye in the liquid crystal in order to avoid deposition thereof. Accordingly, the dye cannot be added in an amount sufficient to satisfy the object of the addition. For example, in the case of the NK-2772 dye described above, the upper limit of addition is 0.4% by weight for the working temperature of the liquid crystal ranging from the temperature for the formation of the smectic phase and that for the liquid phase. In the liquid crystal cell, a bias temperature of about 53.degree. to 54.degree. C. is applied so that the phase transition may be achieved at a low writing power. However, if the display device is left in its non-operational state at a temperature lower than the bias temperature, such as room temperature, the dye may possibly be deposited out of solution. Thus, the the amount of dye added is further limited to about 0.35% by weight.
Consequently, the added amount of dye cannot be increased enough in view of the restriction of its solubility in the liquid crystal.